There is a mystery kidney disease that for years has been killing thousands of farmers in Sri Lanka's rice basket — and as it spreads, locals are becoming increasingly fearful.

In this July 16, 2013 photo, Kumaradasa, a Sri Lankan farmer suffering from a chronic kidney disease of unknown etiology, bathes helped by his wife outside their house in Medavachchiya, Sri Lanka. The cause of his disease, which affects anywhere from an estimated 70,000 to 400,000 people in Sri Lanka's rice basket, remains an enigma without a name. Kumaradasa died on June 8, 2014. (AP Photo/Eranga Jayawardena)

In 20 years the disease has killed up to 20,000 people and sickened up to 400,000 more; some villages report it causes as many as 10 deaths a month.

As the disease progresses and kidneys fail, sufferers become unable to pass liquids on their own and are prohibited from drinking more than just a bottle of water a day — their extremities retain water and become fat with fluid.

Its cause is not yet known, though there are long-held suspicions that agrochemicals are to blame, and Sri Lanka is lacking in the dialysis required to effectively treat the many sufferers.

Though no cases have been reported outside of the North Central province's dry zone, where farming was transformed in the 1960's and 70's following the introduction of modern techniques, the disease has slowly spread from two districts to seven.

A report published by the World Health Organisation two years ago could not find a clear cause for the disease, though it pointed to cadmium, pesticides and other factors, such as arsenic, as possibilities.

Similar diseases are wiping out thousands of farmers in parts of Central America, India and Egypt — and agrochemicals are widely blamed there as well.

Questions remain over whether heavy metals could be leaching into the soil and groundwater from pesticides and fertilizers, which have been found to have high levels of cadmium in previous studies.

Sri Lanka's Agriculture Ministry claims samples are regularly tested and come back within permitted limits — which are much stricter than those in neighbouring countries.

Still, farmers douse their fields with too many chemicals, often using concoctions that include kerosene and multiple poisons. Most do not wear protective gear.

The country of 20 million, which emerged from a quarter-century of civil war in 2009, has just 183 dialysis machines, forcing most villagers to receive less than the three recommended weekly treatments.